The present invention relates to a layered semiconductor light emitting device including a plurality of layers of thin-film semiconductor light emitting elements that emit lights, and an image forming apparatus in which a plurality of such layered semiconductor light emitting devices are integrated.
In order to achieve a thin display, a liquid crystal display (LCD) system and a plasma display system have been developed. The LCD system includes an LCD panel that forms an image or the like by modulating a light emitted by a backlight. The plasma display system includes a plasma display panel of self-luminous type. Both display systems respectively have advantages in terms of brightness, contrast, operation speed, viewing angle, electricity consumption, downsizing, precision or the like, but still have problems to be solved.
In order to solve the problems of these display systems, an organic electroluminescence (i.e., an organic EL) display system and a single-crystal light emitting diode (i.e., a single-crystal LED) display system have attracted attention.
Both of the organic EL display system and the single-crystal LED display system have light sources of self-luminous type, and therefore sufficient properties can be obtained in terms of brightness, contrast, operation speed, viewing angle and electricity consumption. Further, the organic EL display system and the single-crystal LED display system can have simpler configurations compared with the LCD system and plasma display system, and therefore have advantages in reducing size and weight. Further, the organic EL display system and the single-crystal LED display system are expected to be applied to flexible display devices due to their simple structures.
The organic EL display system is advantageous in terms of manufacturing process in that light emitting elements can be formed using printing technique with high precision. That is, a high-precision full-color organic EL system can be manufactured by precisely forming the light emitting elements of red (R), green (G) and blue (B) two-dimensionally. However, the organic EL display system has low light-output efficiency and low reliability compared with the single-crystal LED display system.
In contrast, the single-crystal LED system has high light-output efficiency and high reliability compared with the organic EL display system. However, the single-crystal LED system generally includes cannonball-shaped LED modules for respective pixels, and therefore the size of the single-crystal LED system increases with the number of pixels. In order to manufacture a small-sized display system, it is possible to use bare-chip type single-crystal LEDs. However, even in such a case, the respective bare-chips (i.e., the single-crystal LEDs) need to be diced from a wafer, bonded onto a substrate, and electrically connected using wire bonding, and therefore the manufacturing process becomes complicated. Further, on manufacturing the full-color display system, respective pixels need to be formed by two-dimensionally arranging light emitting elements of red (R), green (G) and blue (B). Therefore, a high resolution and high precision full-color display system is technically difficult to achieve.
For these reasons, Japanese Patent Publication No. 2007-273898 discloses a high-precision full-color LED array using single-crystal LEDs, in which respective light emitting elements of red (R), green (G) and blue (B) are integrated in small areas.
The publication discloses the LED array in which bare-chip type single-crystal LEDs of red (R), green (G) and blue (B) are layered in a direction perpendicular to a light emitting surface. In other words, the LEDs of respective colors are three-dimensionally integrated in small areas. With such a configuration, a high precision image forming apparatus using the single-crystal LEDs having high light-output efficiency and reliability can be obtained.
However, in the LED array disclosed in the above described publication, bonding electrodes need to be formed on a top surface or a bottom surface of each LED. The bonding electrodes of respective LEDs are bonded to each other. Therefore, air layer (with a thickness corresponding to that of the bonding electrode) may be formed between the LEDs. Such air layer causes a difference in refractive index when the light emitted by the LED proceeds into the air layer. Therefore, light-output efficiency of the LEDs decreases. Further, the air layer may be formed at random in between the LEDs, and therefore an amount of leakage light that proceeds laterally in the air layer may increase. As a result, combined light-output efficiency of the LEDs may decrease. Further, when the LEDs are integrated three-dimensionally at high density, the leakage light (proceeding laterally in the air layer) may interface with the light emitted by the adjacent LED, and may change color tone of the light. Furthermore, the bonding electrodes may block the light emitted by the LEDs, with the result that light-output efficiency may decrease.